Problem: Solve for $x$ and $y$ using substitution. ${4x+y = -5}$ ${y = -5x-4}$
Since $y$ has already been solved for, substitute $-5x-4$ for $y$ in the first equation. ${4x + }{(-5x-4)}{= -5}$ Simplify and solve for $x$ $4x-5x - 4 = -5$ $-x-4 = -5$ $-x-4{+4} = -5{+4}$ $-x = -1$ $\dfrac{-x}{{-1}} = \dfrac{-1}{{-1}}$ ${x = 1}$ Now that you know ${x = 1}$ , plug it back into $\thinspace {y = -5x-4}\thinspace$ to find $y$ ${y = -5}{(1)}{ - 4}$ $y = -5 - 4$ $y = -9$ You can also plug ${x = 1}$ into $\thinspace {4x+y = -5}\thinspace$ and get the same answer for $y$ : ${4}{(1)}{ + y = -5}$ ${y = -9}$